Conventionally, there is a lighting device including three solid state light emitting element groups irradiating light of different chromaticities (a red color, a green color and a blue color), each having a lighting control unit to control lighting of the corresponding solid state emitting element groups (see, e.g., Japanese Patent Application Publication No. 2010-176984). The lighting device varies a chromaticity of a mixed color light from the light emitting element groups by controlling respective outputs from the three solid state light emitting element groups and controlling an output ratio thereof. Further, there is a lighting device in which an illuminance (output) of a mixed color light and a chromaticity of a light can be varied at the same time to obtain a comfortable illumination light.
In the conventional lighting devices, the lighting control unit includes a drive circuit for supplying a current to the solid state light emitting element group, and a control circuit for controlling the drive circuit. Further, in order that the mixed color light of the lights emitted from the light emitting element groups has a desired chromaticity, it is necessary to control the output ratio of the light emitting element groups to a target output ratio. To that end, each lighting control unit performs a feedback control such that the current being supplied to the solid state light emitting element group becomes a preset target value. Accordingly, the output ratio of the light emitting element groups is equal to the target output ratio, and the mixed color light has the desired chromaticity.
For example, the chromaticities of the lights irradiated by the respective three solid state light emitting element groups have a color coordinate Pr (red), a color coordinate Pg (green), and a color coordinate Pb (blue) in an xy chromaticity diagram of an XYZ color system shown in FIG. 18. In this case, by setting the output ratio of the light emitting element groups to the target output ratio, the chromaticity of the mixed color light can have a color coordinate P0 on the black body locus BL.
As mentioned above, in order that the mixed color light of the lights emitted from the light emitting element groups has a desired chromaticity, it is necessary to make the output ratio of the light emitting element groups equal to the target output ratio. However, it is concerned that the current being supplied to the solid state light emitting element group is deviated from a target value due to, e.g., a difference in parts used in the drive circuit or control circuit. Accordingly, the output of the solid state light emitting element group may be varied and, thus, the output ratio of the light emitting element groups may be deviated from the target output ratio.
For example, if the currents being supplied to the solid state light emitting element groups are deviated from the target values so that a red light increases, a green light decreases, and a blue light increases, for example, the output ratio of the light emitting element groups is significantly deviated from the target output ratio. Thus, the chromaticity of the mixed color light is significantly deviated from the desired chromaticity and, thus, the color reproducibility of the mixed color light is reduced.
In particular, if a deviation duv from the black body locus BL (hereafter simply referred to as deviation duv) is large, the chromaticity of the mixed color light becomes significantly different from the desired chromaticity. The deviation duv is varied depending on a deviation of an output ratio with respect to the target output ratio of two solid state light emitting element groups irradiating a red light and a green light, wherein the black body locus BL is located between the color coordinate Pr of the red light and the color coordinate Pg of the green light.
As shown in FIG. 19, if an output fluctuation has occurred with respect to each of target values due to an increase in the red light, an increase in the green light, and a decrease in the blue light, the mixed color light has a color coordinate P1. Further, if an output fluctuation has occurred with respect to each of the target values due to a decrease in the red light, a decrease in the green light, and an increase in the blue light, the mixed color light has color coordinates P2.
In this manner, if the output fluctuations in the red light and the green light have occurred in the same direction with respect to the target values, the deviation of the output ratio of the red light to the green light from the target output ratio is small. Therefore, in this case, the chromaticity deviation duv of the mixed color light is small, that is, the chromaticity of the mixed color light is not significantly different from the desired chromaticity (a color coordinate P0).
However, if an output fluctuation has occurred with respect to each of the target values due to an increase in the red light, a decrease in the green light, and a decrease in the blue light, the mixed color light has a color coordinate P3. If an output fluctuation has occurred with respect to each of the target values due to an increase in the red light, a decrease in the green light, and an increase in the blue light, the mixed color light has a color coordinate P4. Further, if an output fluctuation has occurred with respect to each of the target values due to a decrease in the red light, an increase in the green light, and an increase in the blue light, the mixed color light has a color coordinate P5. If an output fluctuation has occurred with respect to each of the target values due to a decrease in the red light, an increase in the green light, and a decrease in the blue light, the mixed color light has a color coordinate P6.
In this manner, if output fluctuations in the red light and the green light have occurred in different directions with respect to the target values, the deviation of the output ratio of the red light to the green light from the target output ratio is large. Therefore, in this case, the chromaticity deviation duv of the mixed color light increases, that is, the chromaticity of the mixed color light is significantly different from the desired chromaticity (color coordinate P0).